1. Field of the Invention
This invention relates to a method for forming a hole inclined relative to the direction of thickness of a workpiece by light energy. More particularly, it relates to a method for forming a hole in which an irregular surface portion is formed in an energy-concentrated portion of the workpiece so that the light energy may be prohibited from being reflected even in case of the incident angle of the light energy exceeding the critical angle for allowing to form a hole with a larger angle of inclination relative to the direction of thickness of the workpiece.
2. Related Art
Recently, a so-called on-demand type printing apparatus, dispensing liquid ink drops from a nozzle responsive to recording signals for recording on a recording medium, such as paper or films, has rapidly come into popular use since it lends itself to reduction in size and production costs.
Meanwhile, a demand is recently increasing for outputting not only letters or pictures but also colored natural pictures similar to photographs along with the letters or figures. In keeping with this demand, it has been a desideratum to print a high-quality natural picture, and reproduction of a half-tone has become critical.
For enabling reproduction of the half-tone, the present inventors have proposed a printing head in which the density of dots printed may be controlled by discharging an ink solution obtained on mixing the ink and a diluting solution and by varying the density of the ink solution for printing the natural picture without deteriorating the resolution.
Such printing head is made up of a portion adapted for applying the force of discharging the ink or the diluting solution to the ink or the diluting solution, such as an ink reservoir or a diluting solution reservoir, having a piezoelement or a heating element, and a nozzle portion (so-called orifice plate) for guiding the discharging of the ink or the diluting solution.
The orifice plate is formed with a through-hole operating as an ink nozzle and another through-hole operating as a nozzle for a dilution solution. These two through-holes are of extremely small diameters.
These through-holes are difficult to form with a boring machine or a drill. There is a limit to the minimum size or diameter of the opening of the through-holes that can be formed utilizing a boring machine or a drill. Additionally, an accurate opening size cannot be maintained if plural through-holes are to be formed. This is caused by wear produced in the cutting chips of the boring or the drilling tool. The through-holes are also difficult to form using an ultrasonic cutting method.
It may be envisaged to form these through-holes by techniques such as reactive ion etching (RIE) or ion milling employed in a semiconductor production process. These techniques are however inappropriate since a workpiece of a thickness sufficient for use as an orifice plate can be worked by this technique only by an extremely time-consuming operation.
The present inventors have proposed in Japanese patent Application No. 7-88999 an orifice plate in which at least one of a through-hole operating as an ink nozzle and a second through-hole operating as a through-hole for the dilution solution are formed obliquely relative to the direction of thickness of the orifice plate. It is however extremely difficult to form the through-hole obliquely by a method employing a boring machine or a drill or by an ultrasonic method.
Thus the through-hole in an orifice plate, which is small in diameter and which needs occasionally to be formed obliquely, is formed by illuminating the light energy, such as that of a laser. Since the laser can be reduced in beam diameter and radiation diameter, it is suited to formation of a hole with a small diameter. As laser, carbon dioxide gas laser or a YAG laser may be employed in addition to an excimer laser.
In laser working, a laser beam having the same angle of inclination as that of the through-hole to be formed is illuminated upon the workpiece surface for forming the through-hole in the proceeding direction of the laser beam.
However, if the through-hole is of an excessively large angle of inclination, laser working also becomes impossible. It is assumed that a laser beam indicated by arrow L is radiated into a first medium having a refractive index n1, so that the laser beam enters a second medium having a refractive index n2, as shown in FIG. 23. If the angles of inclination of the laser beam L in the first medium and in the second medium are .theta. and .alpha., respectively, the equation sin.theta./sin.alpha.=n1/n2 holds. If the angle of inclination .alpha. is 90.degree., the laser beam L undergoes total reflection. That is, sine.theta.=n1/n2, such that it becomes impossible to form a through-hole having an angle of inclination satisfying this equation, that is an angle of inclination being not less than .theta., with the laser beam. That is, if a laser beam indicated by arrow L1 in FIG. 24 is radiated from a surface 101a of a workpiece 101, the laser light is reflected by the surface 101a, as indicated by arrow L2 in FIG. 24.